The reactivity of the C=C double bond allows us to easily test for alkenes in the laboratory, and it also forms the chemical basis for the production of addition polymers, which are key materials in modern society.
🔑 Key Principle
Addition polymerisation is a process in which thousands of unsaturated monomer molecules join together by opening their double bonds to form long saturated polymer chains. No other products are formed during this reaction.
Testing for Alkenes: Unsaturation
The standard laboratory test to identify the presence of a carbon-carbon double bond (unsaturation) is the bromine water test:
- Reagent: Bromine water (\( \text{Br}_2(\text{aq}) \)), which is an orange-brown solution.
- Procedure: Add the test liquid to bromine water and shake.
- Result for Alkenes: The orange-brown solution turns colourless. The bromine adds across the double bond to form a saturated, colourless dihalogenoalkane.
- Result for Alkanes: The solution remains orange-brown (no reaction occurs because alkanes lack a reactive pi bond).
A long-chain saturated molecule formed by the joining together of many unsaturated monomer molecules without the loss of any other atoms or molecules.
The specific arrangement of atoms in a polymer that is repeated many times to construct the entire macromolecule.
Addition Polymerisation
During addition polymerisation, the pi bond of each alkene monomer breaks. The electrons are used to form new sigma covalent bonds between adjacent monomer units. This links the monomers together into a long saturated polymer chain.
Naming Addition Polymers
Addition polymers are named by placing the name of their monomer in brackets and prefixing it with the word poly. For example:
- Ethene monomer forms poly(ethene).
- Propene monomer forms poly(propene).
- Chloroethene monomer forms poly(chloroethene), also commonly known as PVC (polyvinyl chloride).
- Phenylethene monomer forms poly(phenylethene), commonly known as polystyrene.
When drawing a repeat unit, ensure that the extension bonds (representing the continuation of the polymer chain) extend clearly through the square brackets on both sides. Also, remember to write the subscript 'n' at the bottom-right corner. Never draw double bonds inside the bracket of a repeat unit.
Step 1: Rearrange the monomer to focus on the double-bonded carbons. Place the double bond in the centre, horizontally. Put the other groups pointing straight up or down:
- Carbon 1 has two hydrogen atoms attached.
- Carbon 2 has one hydrogen atom and one methyl (\( -\text{CH}_3 \)) group attached.
Step 2: Change the double bond into a single bond and draw horizontal extension lines extending out of both carbon atoms.
Step 3: Place square brackets around the repeat unit, with the extension lines crossing through the brackets. Add the subscript 'n' on the bottom right.
\( -[\text{CH}_2-\text{CH}(\text{CH}_3)]_n- \)
Disposal and Environmental Impact of Polymers
Addition polymers are highly useful materials, but their stability poses significant environmental challenges upon disposal. Because they are saturated hydrocarbons, their molecules consist of strong, non-polar \( \text{C}-\text{C} \) and \( \text{C}-\text{H} \) single bonds.
🔑 Key Principle
Addition polymers are non-biodegradable because they are chemically inert. They contain no polar bonds (like esters or amides) that can be attacked by nucleophiles or broken down by microbial enzymes in the environment.
There are three main methods of polymer disposal, each with distinct advantages and disadvantages:
| Disposal Method | Advantages | Disadvantages / Environmental Concerns |
|---|---|---|
| Landfill |
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| Incineration |
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| Recycling |
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If asked how toxic \( \text{HCl} \) emissions are prevented during the incineration of PVC, state that the waste gases are passed through base scrubbers. The acidic \( \text{HCl} \) gas is neutralised by reacting it with an alkali or base, such as calcium oxide (\( \text{CaO} \)) or calcium carbonate (\( \text{CaCO}_3 \)).
1. Biodegradability: Addition polymers are chemically inert because they are saturated macromolecules composed of strong, non-polar C-C and C-H single covalent bonds. They lack polar bonds or reactive functional groups (such as ester or amide linkages) that are required for hydrolysis or attack by microbial enzymes.
2. Sorting: Sorting is necessary because different polymers have different chemical structures and melting temperatures. If they are melted together, they do not mix successfully and produce a low-strength, brittle mixture of poor quality that cannot be used. Sorting also prevents contamination.
3. Toxic Gas from PVC: Burning poly(chloroethene) releases acidic hydrogen chloride (\( \text{HCl} \)) gas, which is toxic and causes acid rain.